A formidable and virtually unique challenge in influenza vaccine development is that the human population is repeatedly exposed to changing influenza viruses, which render previously immune individuals vulnerable to newly emerged strains, thus causing annual epidemics or even pandemics of influenza. While the human immune system has the intrinsic capability of coping with highly diversified viral antigens with its enormous antibody repertoire (estimated at a magnitude of 1011), it is our hypothesis that this potential diversity is significantly restricted in certain age groups, such as the elderly and the very young, and is not fully realized following immunization with certain types of vaccine in some age groups, such as the trivalent inactivated influenza vaccine (TIV) compared to the live attenuated influenza vaccine (LAIV) in young children^ In this application we propose to. address these, issues using, both serologic and molecular approaches, with an emphasis on the age groups most vulnerable to influenza morbidity and mortality - very young children and the elderly. The specific aims of the proposal are: A1. To compare the antibody responses against the vaccine strains (homotypic reactivity) and mismatched virus strains (heterovariant reactivity) after immunization with LAIV or TIV. We will characterize and compare the heterovariant antibody responses in a collection of paired serum samples from young children, healthy adults and elderly who are vaccinated with these two types of influenza vaccines. We will also clone and express immunoglobulin (Ig) genes from individual antibody-secreting cells (ASCs) elicited shortly after immunization with TIV or LAIV in the different age groups and compare the specificity and affinity of these monoclonal antibodies against both homotypic and heterovariant influenza viruses. A2. To identify factors affecting the sequences of Ig genes encoding influenza-specific antibodies and determine the relationship between specific Ig gene sequence usage and function, we will carry out a systematic analysis of the sequences of Ig genes isolated from ASCs after immunization with LAIV or TIV, and relate the sequence characteristics of Ig genes to antibody reactivity against different influenza strains. In particular, we will compare the Ig gene sequences between the elderly, younger adults and children, between the recipients of LAIV and TIV and between IgG and IgA isotypes.